1. Field of the Invention
The present invention relates to an apparatus, system, and method for lateral two-terminal nanotube devices. More particularly, the present invention relates to an apparatus, system, and method for providing lateral two-terminal nanotube devices configured to capture and generate energy, to store electrical energy, and to integrate these functions with power management circuitry.
2. Description of the Related Art
The limitations of conventional devices for energy capture and storage are well known. In semiconductor p-n junction solar cells, planar device layers typically create only a single depletion layer over the surface to separate photo-induced carriers. As a result, each substrate of a semiconductor p-n junction solar cell can be limited to only a single active layer. Furthermore, some of the semiconductor material can absorb light, producing excitations outside a depletion range. This can prevent the separation of positive and negative charges and the collection of harvested light energy.
Currently, alternate solar cell structures are being explored that utilize nanocomposite structures that mix nanoparticles, such as C-60 or carbon nanotubes, with organic materials having random spatial distributions on a nanoscale. These nanocomposite structures can provide a high density of interfaces between the component materials, effectively enhancing the active regions, analogous to depletion regions in p-n junction semiconductor structures, where charge separation can occur.
However, the nanoscale randomness of the component materials can impede efficient collection of charges at micro- or macro-scale external contacts where charge should be produced, for example, through high electrical resistance through which the charge reaches the contacts. Additionally, these materials, such as conducting polymers, can have relatively high resistivity, further diminishing the efficiency of charge collection at the external contacts.
Furthermore, the limitations of conventional capacitor and battery devices are also well known. Charge storage devices can exhibit similar limitations experienced by conventional solar cell devices discussed above. Electrostatic capacitors that store charge at the surface of electrodes typically do not achieve high areal densities of the electrodes. Electrochemical supercapacitors and batteries that store charge inside their active surfaces and at the surface also can experience similar limitations experienced by conventional solar cell devices discussed above. While sub-surface charge storage can enhance energy density, ion/charge transport into these materials can limit available power.
A number of nanostructures have been explored to improve the power and energy density of conventional solar cell devices and conventional capacitor and battery devices, primarily exploiting higher surface area densities per unit volume of material used in these devices. For example, a high density of nanowires on a surface can substantially enhance the surface area available, producing higher charge density per unit planar area. Furthermore, nanowire and nanotube structures can present shortened pathways for ion transport into the surface, thereby increasing power density. These advancements in technology promise improvements in energy devices, particularly if nanostructures can be formed with sufficient control at the nanoscale to realize functioning and reliable aggregation of massive arrays of nanostructures into larger working devices addressed at the macro- or micro-scale external contacts.
Nanotechnology provides new options for meeting these requirements, particularly using self-assembly phenomena and self-alignment to build more complex nanodevices from simpler nanostructures. For example, anodic aluminum oxide (AAO) can achieve highly regular arrays of nanopores through specific recipes for anodic oxidation of aluminum. Nanopores in AAO may have uniform size and spacing in a hexagonal pattern.
FIG. 1 is a scanning electron micrograph of conventional anodic aluminum oxide nanopore arrays. As illustrated in FIG. 1 in a top view (a) and a side view (b) of the conventional anodic aluminum oxide nanopore arrays, tops of nanopores 1 and 2 each can provide access to narrow columns 3 that can include high aspect ratio nanopore structures. AAO nanopores can be prepared with diameters from 15 to 300 nm, depending on the choice of electrochemical conditions and sequences used during anodization. For nanopores approximately 70 nm in diameter, their center-to-center spacing can be in the order 100 nm. Since the nanopores may be formed to tens of millimeters in depth, very large aspect ratios (depth/diameter) as high as 1000 can be achievable. Furthermore, the density of nanopores, for example 1010/cm2, can ensure very large active surface areas per unit area. Typically, this area enhancement can be as high as approximately 500× planar area. Since wet processing can be used, costs associated with vacuum and gas handling technologies can be avoided, and manufacturing costs could be modest. Thus, AAO can provide a cheap and attractive platform for high density nanostructures and devices made from them. A particular advantage of AAO nanostructures can be that massive arrays can be fabricated with a high degree of control over their shape and spatial relationship, including their depth, width, and vertical shape (all controlled by anodization conditions). The regularity which results is ultimately of major value for manufacturability, providing predictability for properties for the full array. The nanopore arrays can have dimensions comparable to that produced by costly, sophisticated lithography and etching processes in the formation of dynamic random access memory capacitors. However, natural self-assembly from the anodization process itself produces the structures without need for such complex manufacturing steps.
Deposition techniques capable of introducing materials for electrical devices into very high aspect ratio nanopores are limited. Physical techniques, such as evaporation and sputter deposition, cannot sufficiently penetrate deeply into the pores, but chemical methods are suitable. Electrochemical deposition, carried out in electrolytic solutions, can successfully cope with the high aspect ratio because electric fields are established between a bottom region of the pore and a counter-electrode removed from the pore in the electrolyte.
FIG. 2 is a schematic view of conventional anodic aluminum oxide nanopores used to create coaxial nanowires. In particular, FIG. 2 illustrates coaxial nanowires formed in AAO nanopores by electro-deposition, including active storage material 1 at the center (MnO2) surrounded by conducting polymer material 2 (PEDOT) to transport charge efficiently to all portions of the MnO2 charge storage electrode. AAO template material 3 can remain near the bottom of the nanowires to retain the array structure, but can be removed above 4 to expose maximum surface area of the nanowires.
Chemical vapor deposition (CVD) is the dominant method for introducing materials into deep, high aspect ratio pores in semiconductor technology, forming the basis for manufacturing of dynamic random access memory capacitors. As semiconductor device technology has faced even more stringent demands for filling narrower, higher aspect ratio pores or trenches, ALD has emerged with unprecedented ability to coat ultra-thin layers of material uniformly in very narrow, very high aspect ratio, 50-100 or more, structures. A close relative of CVD, ALD can utilize self-limiting adsorption and reaction of CVD precursor molecules in alternating sequences to achieve uniform atomic layer thicknesses deep into the nanopores. Thus, ALD can be an ideal candidate for fabrication of AAO-based electrical nanodevices.
At the same time, the high conformality of ALD has its limitations. Higher doses of ALD gases are required for each atomic layer to coat deeper regions of the nanopore. Thus, ALD process recipes can be chosen to fully coat nanopore sidewalls and bottom regions, or alternatively, to coat the sidewalls to a specific depth short of the bottom of the nanopore. This feature offers major advantages in some device configurations in allowing an external contact made at one end of the nanopore to electrically connect to only one material.
FIG. 3 is a transmission electron micrograph illustrating a conventional nanotube created within and release from an aluminum oxide nanopore. FIG. 3 further illustrates a nanotube made by ALD HfO2 deposition into an AAO nanopore template and subsequent removal of the AAO material to allow the resulting ALD nanotube to be observed in transmission electron microscopy. Darker portions 1 in the images indicate the outer and inner diameter at the ALD nanotube sides, while lighter portions 2 therebetween reflect attenuation by the walls of the top and bottom regions.
The combination of self-assembled AAO nanopores and self-aligned, self-limiting ALD can enable the fabrication of energy devices within the AAO nanopores. Using vertical nanopores that are formed by anodic oxidation of an aluminum thick film, metal-insulator-metal (MIM) electrostatic capacitors have been fabricated, as described by Banerjee et al. (Nature Nanotechnology, submitted for publication).
FIGS. 4a, 4b, and 4c each illustrate the investigated MIM structure investigated. In particular, FIG. 4a is a schematic view of a conventional metal-insulator-metal nanocapacitor structure fabricated by multiple atomic layer deposition steps in anodic aluminum oxide nanopores. As seen in the schematic of FIG. 4a, anodic oxidation of aluminum 1 can cause formation of aluminum oxide Al2O3 2 with deep pores on whose surfaces a sequence of ALD layers can be deposited to create MIM device structure 3. The detailed structure of MIM layers is seen by scanning electron microscopy in FIGS. 4b and 4c for regions at the top and bottom of the nanopores, respectively. The pore diameter was 60 nm, the bottom TiN electrode thickness was 5.6 nm, the Al2O3 dielectric thickness was 6.6 nm, and the top TiN electrode thickness was 12.6 nm, nearly filling the nanopore. It should be noted that layer thicknesses could be readily adjusted to fully fill the nanopore or instead to leave internal volume. The pore depth for the structures shown in FIGS. 4a, 4b, and 4c was 1 mm.
Further, Banerjee, et al. investigated MIM nanocapacitor arrays for both 1 and 10 μm pore depths, forming capacitors whose macroscopic external contacts to the TiN ALD layers in the MIM structure were made above the nanopores and to the underlying aluminum below the nanopores. Capacitors with 0.01267 mm2 area (about 0.1 mm in diameter) connected approximately 106 nanocapacitor structures like those in FIGS. 4a, 4b, and 4c in parallel and indicated capacitance densities of 9 and 90 μF/cm2, respectively. This corresponds to an energy density of order 0.7 W-h/kg, placing the performance of these devices well above the energy density of conventional electrostatic capacitors, while retaining comparable power. While further work is needed to improve leakage current levels and to address other issues, this work on vertical capacitor devices demonstrates the high potential of combining AAO and ALD technologies to create nanostructures optimized for energy device applications.
The drawbacks of such vertical nanodevice structures are (1) their difficulty in their fabrication and (2) the challenge of scaling to higher performance. High quality nanopore arrays can require relatively thick films of aluminum, micrometers in depth to obtain controlled dimensionalities as seen in FIG. 1. Furthermore, patterning of nanopore arrays and devices to construct micro- and macro-scale devices for external contacts can require a sequence of conventional device fabrication steps normally performed on semiconductor wafers or other flat substrates. The thick aluminum films required could necessitate very long deposition times if formed by thin film deposition. Instead, Banerjee et al. developed an anodic bonding technique for bonding the initial aluminum film to a substrate to facilitate subsequent device fabrication steps, for example, making test capacitors. These problems increase if nanopore depths are to be scaled more aggressively for higher performance devices, for example, deeper nanopores. Given the difficulty in creating thick aluminum starting films for vertical nanopores, one may recognize that the vertical technology does not scale to multiple layers on top of each other, as would be desired to increase capacitance density or other energy-related functionality. This also could preclude construction of heterogeneous integrated systems, for example, combining solar or thermoelectric energy capture or generation with electrical energy storage.
Accordingly, what is needed are an apparatus, system, and method for a lateral two-terminal nanotube device configured to capture and generate energy, to store electrical energy, and to integrate these functions with power management circuitry.
Furthermore, an apparatus, system, and method are needed for lateral two-terminal nanotube devices that utilize nanostructures having nanopores formed by anodic oxidation of aluminum, and thin films deposited by atomic layer deposition and electrochemical deposition to form devices in the nanopores. Further, what is needed are an apparatus, system, and method where the nanostructures are coupled to one another to form larger assemblies suitable for power and energy systems.
Furthermore, an apparatus, system and method are needed for lateral two-terminal nanotube devices for capture, generation and storage of energy based on multi-component materials contained within lateral nanoscale pores in aluminum oxide or another dielectric material. Further, what is needed are an apparatus, system, and method where a plurality of lateral two-terminal nanotube devices are wired in parallel to capture energy from light, either solar or ambient, and to generate energy from temperature gradients sensed by thermoelectric devices.
Furthermore, what is needed are an apparatus, system, and method for electrostatic capacitors, electrochemical capacitors, and batteries for energy storage, whereby device layers for energy capture, generation, or storage, can be combined one on top of another or laterally to provide enhanced functionality, including energy and power management systems and electrical power management circuitry with components for capture, generation, storage and distribution.
No prior arrangements have provided an apparatus, system, and method for a lateral two-terminal nanotube device configured to capture and generate energy, to store electrical energy, and to integrate these functions with power management circuitry.